Integrated circuits are typically designed using a library of cells. Each cell has a the circuitry for a commonly used component such as a logic device, a flip-flop, a latch, a comparator, an arithmetic unit, a buffer or set of memory registers, a delay gate, or another component. The cells are arranged and ordered on the silicon and then connected together in order to design the desired circuits. The cells may be limited to very simple devices, such as buffers and logic gates or more complex devices such as multipliers and memory arrays, depending on the library.
Each cell in a library is designed to fit within common physical and electrical standards so that the circuit designer is able to lay any cell in any desired location to build the desired circuit. Typically a standard cell size unit is used for all of the cells. Some cells are then designed as being double height or double width or both. This allows for more complex components to be combined with simpler components using a single library.
In addition to connections between cells, a cell typically has connections between transistors, diodes, and other components within the cell. These interior connections may be made using metal lines formed over the transistors, diodes, and any other components formed on the silicon substrate of the integrated circuit. The metal lines are formed in layers that may be named M1, M2, M3, etc., depending on the library. The metal lines are designed so that they are in parallel tracks between dielectric lines. This make the circuit easier to fabricate and it makes it easier to connect cells together because the connection points will also be on the parallel tracks.
Advanced process design rules require a specified cut spacing, that is a gap or distance, between the ends of any two metal lines on the same track. The design rules may also require a minimum distance between a via and a non-related line. When vias or metal line ends occur at the top or bottom of a standard cell design, the design rules require that the vias are some distance from the edge and that the metal lines end some distance before the edge of the cell. In this way, if another cell is placed directly above or below and if this cell also has metal lines in the same tracks, then the line ends between the two cells will be spaced sufficiently from each other. The design rules ensure electrical performance near the cell boundaries of two different cells.